The invention relates to a method for measuring the distortion of a high-frequency power amplifier and to a corresponding measuring device. The invention also relates to an equalizing method for automatically equalizing a high-frequency power amplifier and to a corresponding equalizing means.
High-frequency power amplifiers usually do not have an entirely linear characteristic. This causes a distortion of the high-frequency signal that is amplified by the high-frequency power amplifier. Thus, due to the non-linearities of the high-frequency power amplifier, an undesirable noise spectrum emerges.
Given multi-carrier signals such as are used in the digital transmission of audio and video signals, the noise spectrum is expressed in extraband disturbances, which lie outside the transmission band, and intraband disturbances, which lie inside the transmission band. To guarantee a perfect transmission of the multi-carrier signal, the noise spectrum must comprise a predetermined level distance from the useful signal. Therefore, relatively high requirements are set for the linearity of high-frequency power amplifiers in the output stage of an amplifier for DAB (Digital Audio Broadcasting) signals and DVB (Digital Video Broadcasting) signals.
Previously, in the balancing of high-frequency power amplifiers of this type, it was common to feed a multi-carrier balancing signal to the input and to observe the output signal of the high-frequency power amplifier with a spectrum analyzer. In the signal path prior to the high-frequency power amplifier, a predistorting means was arranged, which permits a predistorting of the high-frequency signal that is to be amplified. The predistortion characteristic is defined by a diode resistance network with variable resistances, so that the curve of the predistortion characteristic can be variably adjusted by a plurality of parameters. By varying the adjustment parameters of the predistortion network, it was then iteratively attempted to achieve an optimally large shoulder spacing between the useful signal and the noise signal in the output spectrum of the high-frequency power amplifier. Since there is no strategy for adjusting the individual parameters of the predistortion network, and the adjustments of the individual parameters have a mutual influence on one another, this procedure is extremely time-consuming and thus expensive. Furthermore, an adjustment that has been found is dependent on the temperature, the age of the components of the high-frequency power amplifier, and other influences.